Cylinder bores got egg shaped
 

Dropped off my block to get re-honed for some new rings and the machine shop calls and tells me I have two cylinders that do not look good and can I come in. When I took it apart (wish I had pics of the cylinders) I saw with my horrible eyes two looked kinda bad and was only scuffed on the sides. So the #6 and #4 cylinders need at least twenty thousands to clean up the out of round. The bores are out of round next to each cylinder and #6 has it on both sides and #4 has it on the #6 side. I brought in my pistons and the #4 and #6 were both just over a thousandth smaller than the other pistons even with the line to line coating. This motor only has around 20 hours and is a bowtie block bored at 4.530 currently but going to 4.560. Pistons are Icon forged. When it was assembled the bores looked perfect. I had it apart due to blowby issues and found some bad leak down numbers. This could be one reason. Any ideas on what could of happened.

Whats your rod stroke and compression height? Engine speed and hours?

Got a call from machine shop and it looks like when the block was machined for these piston it was bored to 4.535 which is way too much for the piston and skirt coating I had. This could of been why I had blow by issues? The engine is a 4.25 stroke, 4.385 rod motor, from what see online the compression height on these are 1.270. Probably about 20 hours and did run it quite a few times on length (around 20-30 minutes) at 4500-5000. Icon piston is local to me and have a friend who does all the race stuff there so he got me a new set of forgedpistons again and is talking to my machine shop which happens to be his. So I am sure it will get done right this time.

I think you mean 6.385 rod..................

That’s a very odd size to accidentally bore to. Your engine builder and machine shop both screwed up. If they aren’t making it right find new ones.

It is at a new one now.

so number 4 and 6 were bored different than the rest.

The blocks bore was/is actually measured at 4.530, and what was the pistons measurement? Was this coating applied from ICON or was it done somewhere else? Im curious what the actual piston to wall clearance was. Any chance the original machine shop bored the extra .005 from the cylinder, due to the coating/piston measurement?

Guess what Im asking is, was piston clearance on target, or , did you simply get the pistons hot and scuff the bores up from a bad tune up

Piston to wall clearance
 

just wanna make sure I'm get this correct. The out of round your refeering to on 4 and 6 is from the scuffs on the sides of the walls correct. Also what was original piston to well clearance .005? Icons site recommends like .007 plus for their 2618 piston but would imagine the exact clearance came with pitons. They do have a FHR the was partially forged that I believe they did away with though. Curious what your new builder is claiming for correct clearance. Wish you coulda got some pics. Seems like you have iron heads and 10.5:1 compression however higher altitude if I remember right and had some reversion issues. You ever replace cam with less overlap? Just going by memory but seems like it was around 24-26 deg. Hope the best for you cause I know you have a lot of time invested and now getting pricier.

Clarification, all bores were bored to 4.535 which is odd but the two pistons in the #4 and #6 holes measured over a thousandth smaller that the other 6. The coating was put on by Icon and is the Line2Line coating and these were not the FHR series but the premium forged. I do not know the exact piston to wall clearance but it seems to be a bit much as hardly any pistons had any coating that was abraded. I will call the machine shop today and get more information on the old and new clearances. It is my understanding that the coated pistons are supposed to be installed tight in order for the coating to abrade into the cylinder wall.

In regards to the tune up I know I was pretty fat for a few trips until I got it leaned out finally.

I am probably putting a set of brodix bb2 on now and getting a new cam ground for the new heads. Should still be around 10:1 but will be a 555 now.

Are the bores glazed ? Like mirrors?

How's the wall thickness of that block by opening it up another .030

A machine shop not familiar with the line 2 line coating probably measured the piston over the coating. Added the recommended clearance from the piston manufacture and you ended up with a ton of clearance once the coating wore in. It probably just beat itself to death in the bore.

Not at all, in fact more scratches than I would want. I found some pics

Spoke with the shop and the old setup had over .008 clearance with the coating so over 10 thousands with no coating and caused a lot of piston rock and horrible ring seal. It does look like the block was machined too big to the coating and not the actual piston skirt size. This will not happen this time. Lesson learned and opportunity for more power!

Why did only two cylinders have a problem

I dont think that clearance caused the problem. Sounds to me like the pistons got hot

I agree, looks like the coating has clumped up on the skirt on one piston

I never had any overheating issues with the boat. I never saw the water go over 120 degrees and mostly never even registered due to the cold lake I boat on and oil temp never went above 205. If it did have any heating issues it was on the dyno and I was not there.

I dont' mean overheating water temp. I mean overheating the piston from a lean fuel mix, timing issues, etc. When you heat the piston, it grows, and grows fast.

the scuff marks go all the way up to nearly the deck , and are along the piston pin side . If it was excessive piston rock, you wouldn't see the marks on the pin side of the bores . At least I wouldn't think so.

Did you tune this on the water ? We're u using a wideband or reading plugs ?

If this is carbureted, it is pretty weird to see a lean cylinder 4.

I was tuning on the water and only reading plugs as I do not have a wideband (yet). I know this thing was super fat at the beginning of the summer and eventually I leaned it out over the summer in small increments reading the plugs and going off of the performance of boat. On our last outing of the year I kept putting a degree or two of timing in it to see what I could get out of her on for speed but that was only for a few short pulls. It normally runs at 34 degrees timing.

I'll have to go back and look thru some more of the dyno sheets I have when I get some time, they are buried in a folder somewhere. The 2 that I did find, with an EGT probe in each cylinder, did show #4 and #6 to be the hottest out of all 8 at 4000RPM. By 6000RPM, #2 and #8 were . This was a CNC ported head, NA engine with a custom ported intake manifold. I'd guess that all bets are off when using a vintage set of GM iron rectangles with grossly imbalanced intake ports , and unknown intake/plenum design.

Piston Scuffing
This is a sure sign of overheating and/or insufficient clearance. When an engine runs hot, the pistons swell up reducing the clearance between the piston and cylinder. The cylinder bore can also distort from localized hot spots, further adding to the problem. If a piston gets hot enough, it will scuff the wall wiping material off the sides of the piston. Where this occurs will give you a clue as to the cause.If overheating is involved, the scuffing will be primarily on the upper ring lands and on the sides near the wrist pins. There may also be oil carbon and lacquer burned onto the underside of the piston indicating it was unusually hot.The presence of scuff marks on the lower skirt area would tend to indicate a lubrication problem rather than an overheating problem. Scuff marks on the edges or corners of the thrust sides of the piston would indicate bore distortion as the culprit. Scuffing on both thrust sides would indicate binding in the wrist pin.Any time you find evidence of scuffing, therefore, it’s important to diagnose and correct the underlying cause to prevent a repeat failure.If the engine was recently rebuilt, the scuffing problem may be due to insufficient assembly clearances. The only way to know is to measure the pistons and cylinders. Measure the pistons at the top, center and bottom of the skirt area in two directions (parallel to the wrist pin and perpendicular to the wrist pin) to see if the pistons are within the manufacturer’s specifications.If the pistons are the correct size, then measure the cylinder bore at the top, middle and bottom, also in two directions (parallel to the crankshaft and perpendicular to the crankshaft). This will tell you how much taper is in the bores, and if bore distortion is a problem. By subtracting the piston dimensions from the bore dimensions, you can figure the piston-to-cylinder clearance. If the clearance is within specifications, the underlying problem is overheating.Many aftermarket replacement pistons today are available with moly coated skirts to protect new pistons against scuffing. The coating also allows you to run slightly tighter clearances to reduce piston noise when the engine is cold, and blow-by. Upgrading to coated pistons is probably a good idea for any engine that has a history of scuffing or may be prone to detonation or overheating. Think of it as insurance against piston problems. Back to Basics: Preventing Piston Problems - Engine Builder Magazine

This was merc's spec on the 540ci for piston clearance.

Got some pics of the cylinders including the 2 in question..

widebands are so cheap. Inexcusable to tune with out one.

Any suggestions or have a brand that is preferred? As I have never used one.

they all use the $80 Bosch sensor

Innovate Motorsports Wideband Air/Fuel Ratio Tuning

you do a little searching and you can get them around 50 bucks.

The way I see it is you potentially have a few things going on. Without going into depth as MT and others have covered at some point through the thread. 1 is original machine shop quality of work. Seems like cylinders are out of round and possibly were not chamfered among other things. 2 is how builder didn't catch the two pistons being a .001 smaller shows he didn't throughly check bores, pitons etc prior to assembly. I don't believe the .001 smaller pistons 4 and 6 would cause this but is coincidental. I would do a little research on the icon pistons. Just wonder how much they're really expanding under normal heat conditions. I know they've been used in a lot of builds but... A wideband o2 is definately in order s you know cause like MT said heat is probably the number one cause of scuffing. What carb and jets did you end up with at end of season? On dyno closer to sea level you'd be plenty rich I'd think at least where you started. Just curious how much jet you went down. Id think a couple 2-3 sizes at your elevation would be close assuming your egts on dyno were on target.

I'm sure your new machine shop knows but be sure and sonic test. My NOS GM blocks tested out fine to go out to 4.600 and still plenty safe if were gonna be NA. Only taking them to 4.530 for SC builds.

I'm no expert but looks like to much heat. The last picture of the piston shows scuffing in the center of the piston. If it was a loose/ rocking condition I would think you would have scuffing on the top or bottom of the piston.

Another thing I wanted to add is it's really difficult to determine what cylinders are notoriously hotter. Many factors however most of the BBC's with dart heads I've dyno'd were 8 then 2 but If you google it you'll pretty much cover every cylinder from 1 to 8. To many variables right down to aftermarket heads, porting, stock GM heads, intakes, exhaust and on and on. Even a cam. In a perfect world all would be 100 % efficient and even if each port was to flow identacle you'd still have different temps. Did you purchase FF's heads. Also you still keeping wet exhaust I assume due to already known reasons.

Was your original machine shop a known reputable shop? Seems like he might be in need of some new or updated equipment and also purchase a bore gauge. Lol.. Hey we all have troubles but in the end it's all worth it provided you move forward with confident reputable people and get your o2 wb. You'll be able to go boat with confidence free this is all over.

So the mystery of the egg shape cylinders is revealed. While boring the block they found 2 sleeves. Too thin now and moved out of cylinder n their own. Only .034 after boring.

Glad your moving forward. Are those 1/8 sleeves or did you go thinner. If they are it would bring the bore out to 4.655 if my math is right.

Your math is good as that will be very close I think. The shop actually called me today and they ordered the new sleeves. We had a talk about if this was going to be too big of a bore for the the block but I guess they are proceeding. Two of the guys at the shop offered up Dart blocks while I was there. Was tempted but sleeves are way cheaper than a block.

Just make sure your builder sonic test all the cylinders if he hasn't already. The thickness can vary a lot. Casting flaws, core shifts and scale. Especially if it ever seen salt water.

Opinions will vary from machine shop to builder and individual to individual. HP, stroke etc can determine min also. I'd hate to see anything less than .210 - .215 on the thrust side and maybe .040 less on pin side. Obviously more the merrier. It's to bad that happened to you. Boating can suck but worth it $$$$.

So they ended up sleeving the block and it turned out pretty good and has plenty of meat for sure.

Good deal. One less thing you need to think about. You going to go with wideband o2. Cheap insurance especially if your movig around different altitudes. I went with in innovations # 3844 I think and comes with total of 20 ft cable. You can get for 160.00 + -. Numerous others are out there but I ended up wth this one. In fact recently found as my fiancé stashes all my piles of goodies when company comes over. Hate that. Hah. Anyway congrats.

I am for sure going to use the wideband O2. Might have to bit the bullet and buy some real headers too and will have them built with the bungs.